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91.
92.
Positive interactions among plants 总被引:3,自引:0,他引:3
Experimental evidence for positive interactions, or facilitation, among plants has increased markedly during the last 10 years.
Experiments documenting facilitation have been conducted in many diverse ecological systems, which suggests that positive
interactions may be fundamental processes in plant communities. Here, I review the evidence for facilitation, the mechanisms
by which facilitation operates, and the effects facilitation has on community structure. Facilitative mechanisms may act simultaneously
with resource competition or allelopathy, and the overall effect of one species on another may be the product of multiple,
complex interactions. Positive interactions may also determine community spatial patterns, permit coexistence, enhance diversity
and productivity, and drive community dynamics. Once viewed as anecdotal and idiosyncratic, facilitation is now contributing
to a more complete understanding of community structure and dynamics. 相似文献
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94.
Molecular evolution of mitochondrial 12S RNA and cytochrome b sequences in the pantherine lineage of Felidae 总被引:4,自引:2,他引:2
DNA sequence comparisons of two mitochondrial DNA genes were used to infer
phylogenetic relationships among 17 Felidae species, notably 15 in the
previously described pantherine lineage. The polymerase chain reaction
(PCR) was used to generate sequences of 358 base pairs of the mitochondrial
12S RNA gene and 289 base pairs of the cytochrome b protein coding gene.
DNA sequences were compared within and between 17 felid and five nonfelid
carnivore species. Evolutionary trees were constructed using phenetic,
cladistic, and maximum likelihood algorithms. The combined results
suggested several phylogenetic relationships including (1) the recognition
of a recently evolved monophyletic genus Panthera consisting of Panthera
leo, P. pardus, P. onca, P. uncia, P. tigris, and Neofelis nebulosa; (2)
the recent common ancestry of Acinonyx jubatus, the African cheetah, and
Puma concolor, the American puma; and (3) two golden cat species, Profelis
temmincki and Profelis aurata, are not sister species, and the latter is
strongly associated with Caracal caracal. These data add to the growing
database of vertebrate mtDNA sequences and, given the relatively recent
divergence among the felids represented here (1-10 Myr), allow 12S and
cytochrome b sequence evolution to be addressed over a time scale different
from those addressed in most work on vertebrate mtDNA.
相似文献
95.
Sea urchin Hox genes: insights into the ancestral Hox cluster 总被引:3,自引:0,他引:3
We describe the Hox cluster in the radially symmetric sea urchin and
compare our findings to what is known from clusters in bilaterally
symmetric animals. Several Hox genes from the direct-developing sea urchin
Heliocidaris erythrogramma are described. CHEF gel analysis shows that the
Hox genes are clustered on a < or = 300 kilobase (kb) fragment of DNA,
and only a single cluster is present, as in lower chordates and other
nonvertebrate metazoans. Phylogenetic analyses of sea urchin, amphioxus,
Drosophila, and selected vertebrate Hox genes confirm that the H.
erythrogramma genes, and others previously cloned from other sea urchins,
belong to anterior, central, and posterior groups. Despite their radial
body plan and lack of cephalization, echinoderms retain at least one of the
anterior group Hox genes, an orthologue of Hox3. The structure of the
echinoderm Hox cluster suggests that the ancestral deuterostome had a Hox
cluster more similar to the current chordate cluster than was expected Sea
urchins have at least three Abd-B type genes, suggesting that Abd-B
expansion began before the radiation of deuterostomes.
相似文献
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97.
An exotic grass invades salt marshes of southern California in very wet years and where there are sewage spills or urban runoff. A series of growth-chamber, mesocosm, and greenhouse experiments explored whether soil salinity and/or waterlogging could explain invasion patterns. In all experiments, salinity significantly affected the growth and distribution of Polypogon monspeliensis (rabbit-foot grass, an exotic annual grass) and Salicornia virginica (pickleweed, a native perennial succulent). High salinities caused a greater reduction in seed germination rates for P. monspeliensis than for S. virginica, indicating that high salinity limits establishment and the spread of this exotic grass. At Tijuana Estuary, fresh water inputs to tidal mesocosms lowered soil salinities, increased cover of P. monspeliensis, and decreased cover of S. virginica. Polypogon monspeliensis outcompeted S. virginica under all salinity and hydrology treatments in the greenhouse experiment. Seasonally-low soil salinities caused by winter runoff and anthropogenic fresh water inputs are the likely factors controlling annual variations in the distribution of P. monspeliensis in southern California salt marshes. Our understanding of the causes of invasion is readily applicable to management: local invasions may be reversed by adding salt, and larger scale problems could be avoided by reinstating more natural hydrologic regimes. 相似文献
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